A silica sol in which colloidal silica particles having a particle diameter of about several to 100 nm are dispersed in a liquid has high transparency. Furthermore, due to their high hardness, the colloidal silica particles are mixed into resins and the like to be used as a reforming agent for, for example, providing hardness or heat resistance to the resin. When the colloidal silica particles are mixed with a resin or the like, an aqueous silica sol may be used. However, a sol in which colloidal silica particles are dispersed in an organic compound, such as an organic solvent-dispersed silica sol and a polymerizable compound-dispersed silica sol that is to be the resin raw material, is often used in consideration of its compatibility or its reactivity with the resin or the like.
As an example of the organic solvent-dispersed silica sol, Patent Document 1 discloses a process for producing an organic solvent-dispersed silica sol in which an aqueous silica sol and an organic solvent are mixed and then the mixture is dehydrated with an ultrafiltration membrane. Furthermore, Patent Document 2 discloses a process for producing a silica sol in methanol in which metal ions contained in an aqueous silica sol are removed by ion exchange, next, an organic base is added, subsequently, methanol is mixed, and then the mixture is concentrated and dehydrated by an ultrafiltration method. Patent Document 3 discloses an organic solvent-dispersed silica sol in which an alkaline earth metal ion is bonded on the surface of colloidal silica particles. Furthermore, as the polymerizable compound-dispersed silica sol, Patent Document 4 discloses an oxide sol that includes 80 mol % or more of a reactive monomer having a polymerizable unsaturated bond in the molecule on the basis of the dispersion medium.
When the organic solvent-dispersed silica sol is prepared from an aqueous silica sol as the raw material, alkali metal ions such as Na ions contained in the aqueous silica sol are previously removed by cation exchange to make an acidic aqueous silica sol. The alkaline aqueous silica sol containing the alkali metal ions has poor compatibility with many organic solvents such as alcohols and ethers and thus the mixing with such organic solvents causes the aggregation or the gelation of the colloidal silica particles.
It is known that the acidic aqueous silica sol obtained according to the above process has the characteristics that the alkali metal ions present in the colloidal silica particles are eluted with time to increase the pH, and thus the viscosity of the silica sol increases and finally the gelation occurs. Therefore, disclosed is a process for stabilizing an acidic aqueous silica sol in which an alkaline aqueous silica sol is cation-exchanged, then the obtained sol is aged without treatment or in the presence of a small amount of an acid to diffuse the alkali metal ions in the colloidal silica particles into the colloid dispersion medium, and then the alkali metal ions are further removed by cation-exchange (Patent Document 5). Furthermore, disclosed is a process for producing a stable acidic aqueous silica sol in which an aluminum compound is present in the particle growth step of the colloidal silica particles to contain the aluminum compound in the colloidal silica particles, and thus the aluminum compound can trap the alkali metal ions present in the colloidal silica particles (Patent Document 6).    Patent Document 1: Japanese Patent Application Publication No. JP-A-59-8614    Patent Document 2: Japanese Patent Application Publication No. JP-A-2-167813    Patent Document 3: Japanese Patent Application Publication No. JP-A-2007-63117    Patent Document 4: Japanese Patent Application Publication No. JP-A-62-95130    Patent Document 5: U.S. Pat. No. 3,342,747 specification    Patent Document 6: Japanese Patent Application Publication No. JP-A-63-123807